1. Field of the Invention
The present invention relates to an optical fiber cable (hereinbelow, referred to as xe2x80x9coptical cablexe2x80x9d), in particular, an optical cable employing plastic optical fibers.
2. Discussion of the Background
Optical fibers have been widely employed to transmit high volume of information fast and reliably in recent communication. The optical fibers include silica optical fibers, such as silica single-mode optical fibers, plastic optical fibers (POF) and other fibers. In particular, the plastic optical fibers have a larger diameter than the silica single-mode optical fibers and are excellent in flexibility. From this viewpoint, the optical cables, which employ plastic optical fibers has optical transmission lines, are excellent in workability in end treatment and connection treatment of the optical fibers needed during installation, and in wiring. The optical cables are effective as a short distance trunk in a building after lead-in from a trunk cable, a branch cable, or a line cable for a Local Area Network (LAN) system.
The optical cables are usually configured to cover optical fibers and tensile strength reinforcing members (tension members) for avoiding elongation of the optical fibers due to tension with a sheath. In general, the optical fibers have a primary resin covering applied on a surface to prevent disturbance light from entering, to avoid damage due to a mechanical external force, or for another reason. In the case of optical cables for communication, two or more optical fibers for both input and output are usually housed.
One of the optical cables comprises optical fibers 41a and 41b, primary covering layers 42a, 42b for covering the optical fibers, and a secondary covering layer 43 for covering the optical fibers as shown in FIG. 4(a) for instance (see, e.g., JP-A-11-211954).
The optical cable shown in a schematic sectional view of FIG. 4(b) has a structure wherein two optical fibers 44a and 44b are provided in a cavity 46 delimited by a sheath 45, and tensile strength reinforcing members are embedded in the sheath (see, e.g., JP-U-60-60714).
The light cable shown in a schematic sectional view of FIG. 4(c) has a structure wherein an optical fiber 48 having a surface covered with a primary covering layer 47 is arranged in a cavity 50 delimited by a sheath 49 (see, e.g., JP-A-7-72356).
However, the optical cables that have been proposed or used have raised the following problems:
1) In the cable having the structure shown in FIG. 4(a), a heat test (at 70xc2x0 C. for 24 hours) shows that a resin, such as polyethylene, as the covering material is heat-shrunk to form microvents in the surfaces of the optical fibers, which create a problem of resistance to heat in terms of an increase in attenuation.
2) In the cable having the structure shown in FIG. 4(b), the provision of the plural optical fibers in a single cavity creates a problem of resistance to pressure that, when an external force, which is caused, e.g., when a person steps on the cable, is applied to the cable, the plural optical fibers in the single cavity get in contact with each other, are pressed each other, be squashed each other at the worst, or are subjected to plastic deformation to increase attenuation.
3) In the cable having the structure shown in FIG. 4(c), an increase in attenuation, which is caused by flexing action during bending, can be suppressed by determining the unoccupied ratio of the optical fiber in the cavity at 2-30%. However, there is created a problem of mechanical properties, such as an impossibility in avoiding an increase in attenuation to flexing action during bending since the upper limit of the unoccupied ratio is restricted in terms of connection with an optical connector, which is required when the optical cable is connected to the optical connector.
In particular, a graded refractive index plastic optical fiber (hereinbelow, referred to as xe2x80x9cGI-POFxe2x80x9d), which is prospective as an optical fiber for next-generation communication because of a fast and large volumetric data-carrying capacity, realizes a fast and large volumetric data-carrying capacity by having a refractive index distribution in a sectional direction of the fiber. An optical cable that houses a GI-POF is sensitive to generation of microvents caused by heat-shrinkage of a covering material, application of an external force, flexing action during bending or other factors, and transmission properties are likely to be deteriorated by these disturbances.
The production of an optical cable having a GI-POF is carried out by covering and molding a GI-POF and a structural element, such as a tension member, for protection against tension with, e.g., an extruded thermoplastic resin. There is a possibility that a low molecular chemical compound in the GI-POF is thermally defused by thermal affection from, e.g., the thermoplastic resin molten at a high temperature to change the refractive index distribution of the GI-POF during the production. From this viewpoint, it is necessary to carry out the production so that the GI-POF is not thermally affected during covering and molding.
It is an object of the present invention to provide an optical cable excellent in resistance to heat and mechanical properties to flexing action, and capable of preventing attenuation from increasing.
The present invention provides an optical fiber cable comprising two or more optical fibers and a partitioning spacer housed in a space encircled by a sheath; the partitioning spacer including an axial portion and a plurality of partitioning plate portions; the partitioning spacer having a sectional shape that the partitioning plate portions radially extend toward an inner circumferential surface of the sheath from the axial portion; and each of the partitioning plate portions having a leading end provided with an enlarged portion in contact with the inner circumferential surface of the sheath and a connecting portion connecting the enlarged portion to the axial portion; wherein the space encircled by the sheath is divided into a plurality of partitioned slots by the partitioning plate portions, and the respective optical fibers are distributed so that two or more optical fibers are not provided in a single partitioned slot.
It is preferable that the sectional shape of the partitioning spacer has the following relations (1) and (2) wherein each of the enlarged portion has a maximum dimension L in a direction perpendicular to a radial direction, each of the connecting portion has a length K in the radial direction, each of the connecting portion has a dimension W in the direction perpendicular to the radial direction, and each of the optical fibers has an outer diameter R:
Lxe2x88x92Wxe2x89xa7Rxe2x80x83xe2x80x83(1)
Kxe2x89xa7Rxe2x80x83xe2x80x83(2)
The optical fiber cable is appropriate as a cable with graded refractive index plastic optical fibers (GI-POF) employed therein.
It is preferable that at least one tension member is provided in a partitioned slot without an optical fiber provided therein. At least one of a power line and an information transmission line may be provided in a partitioned slot without an optical fiber provided therein.
It is preferable that the sheath has a hardness of not higher than 95 Shore A hardness. In this case, it is preferable that the sheath is made of thermoplastic resin, and the thermoplastic resin is one selected from soft vinyl chloride, chlorinated polyethylene and soft polyethylene.
The present invention also provides a method for preparing the optical fiber cable stated earlier, comprising distributing the optical fibers in the partitioning spacer, and then forming the sheath by thermoplastic resin extruded from a resin extruder. In the method, it is preferable that the partitioning spacer is heat-treated under a thermal environment at 70-90xc2x0 C. before preparation of the optical fiber cable.
An optical fiber to be accommodated in the optical cable according to the present invention is a bare optical fiber or an optical fiber cord, which is made of a plastic. The optical fiber cord covers one wherein at least one bare optical fiber is covered with a covering (including a ribbon fiber and the like). Although there is no particular limitation on the material for the covering, a thermoplastic resin for covering a bare optical fiber is applicable. Examples of the thermoplastic resin are polyethylene, polyvinyl chloride, polymethyl methacrylate and an ethylene-tetrafluoroethylene copolymer.
Examples of the bare optical fiber are one made of fluororesin, one made of polymethyl methacrylate (PMMA) resin, and one made of polycarbonate resin. Among them, one made of fluororesin or one made of PMMA resin is preferable in terms of excellent transmission performance. One made of fluororesin is particularly preferable since the wavelength of employed light can be selected from a wide range. As the fluororesin, amorphous fluororesin having substantially no Cxe2x80x94H bond is preferable.
As the optical fiber according to the present invention, a multi stepped refractive index optical fiber, a graded refractive index optical fiber and the like are preferable, and a graded refractive index optical fiber is more preferable. A specific example of such a graded refractive index optical fiber made of fluororesin is one disclosed in JP-A-8-5848.
In the optical fiber according to the present invention, a sheath, which encircles a space for housing optical fibers, is preferably made of thermoplastic resin. It is preferable that the sheath has a hardness of not higher than 95 Shore A hardness, in particular a hardness of 70-80 Shore A hardness. Although there is no limitation on the thermoplastic resin, examples of the thermoplastic resin are soft vinyl chloride, chlorinated polyethylene and soft polyethylene. Among them, soft vinyl chloride is preferable in terms of moldability at a low temperature.
A partitioning spacer, which is housed along with the optical fibers in the space encircled by the sheath, includes an axial portion and a plurality of partitioning plate portions. The partitioning spacer has a sectional shape that the partitioning plate portions radially extend toward an inner circumferential surface of the sheath from the axial portion. Each of the partitioning plate portions has a leading end provided with an enlarged portion in contact with the inner circumferential surface of the sheath and a connecting portion connecting the enlarged portion to the axial portion.
The partitioning plate portions provide partitioned slots wherein the optical fibers are distributed. The respective optical fibers are distributed so that two or more optical fibers are not provided in a single partitioned slot. This arrangement can prevent an optical fiber from contacting another optical fiber, and an optical fiber and another optical fiber from pushing against each other. The sectional shape of the partitioning plate portions is appropriately determined, depending on the number of the optical fibers, the outer diameter of the optical fibers, the presence and absence of a tension member, the shape of the respective members, the disposition pattern of the respective members and the like. As specifically shown in FIG. 1 stated later, it is particularly preferable that the sectional shape of the partitioning spacer has the following relations (1) and (2) wherein the enlarged portion has a maximum dimension L in a direction perpendicular to a radial direction (hereinbelow, referred to as xe2x80x9cthe maximum sectional dimensionxe2x80x9d), the connecting portion has a length K in the radial direction, the connecting portion has a dimension W in the direction perpendicular to the radial direction, and an optical fiber has an outer diameter R:
Lxe2x88x92Wxe2x89xa7Rxe2x80x83xe2x80x83(1)
Kxe2x89xa7Rxe2x80x83xe2x80x83(2)
The partitioning spacer may be formed to have the partitioned slots spirally provided therein in the longitudinal direction thereof, i.e., the longitudinal direction of the optical cable. The spiral partitioning spacer offers advantages in that neither difference in length between the inner circumference and the outer circumference nor increase in attenuation is produced at the time of winding the optical cable since the optical fibers are equally located on the outer circumferential side and the inner circumferential side of the optical cable along the spirals of the partitioning spacer.
The partitioning spacer is preferably molded by extrusion since the spacer needs to maintain structural continuity in the sectional shape in the longitudinal direction of the optical cable. As to the material for the partitioning spacer, thermoplastic resin having a relatively low hardness, such as soft (low density) polyethylene or soft vinyl chloride, is appropriate. When a material, which has a stretching ratio of not higher than 2% in the longitudinal direction of the optical cable under a tensile load of 220 N, is employed to form the partition spacer, the partition spacer per se can function as the tension member, which offers an advantage in that the tension member can be eliminated.
From the viewpoint that the thermal hysteresis of the partitioning spacer given at the time of molding the spacer is removed in order to restrain the deformation due to heat given during preparation of the optical cable, it is effective that the partitioning spacer is heat-treated under a thermal environment at 70-90xc2x0 C. before being housed in the spacer encircled by the sheath.
In the optical cable according to the present invention, at least one tension member is provided in a partitioned slot without an optical fiber provided therein to protect the optical fibers against tension of the optical cable, except in cases where the partitioning spacer mainly function as the tension member. There is no limitation on the material for the tension member. Examples of the material are a wire material, such as a metal wire or an FRP wire, or highly stiff continuous fiber, such as aramid continuous fiber. In particular, highly stiff continuous fiber, such as aramid continuous fiber, is preferable in terms of ease in bending of the optical cable and a reduction in damage to the optical fibers when an external force, such as impact or pressure, is applied to the optical cable.
It is advantageous, for simplification in wiring work in a building or another place, that at least of a power line and an information transmission line is provided in a partitioned slot without an optical fiber provided therein. Examples of the information transmission line are a coaxial cable, an unshielded twisted pair wire (UTP) and the like.
The present invention provides a method for preparing the optical fiber cable stated earlier, which is characterized in that the method comprises forming the sheath by a thermoplastic resin extruded from a resin extruder after distributing the optical fibers in the partitioning spacer.